1. Field of the Invention
This invention relates to attitude control systems for three-axis stabilized spacecraft. More specifically, the invention relates to controlling transition from thruster control to momentum wheel control in a momentum-bias three-axis stabilized geosynchronous spacecraft.
One of the tasks in a spacecraft having a momentum-bias attitude control system is control of the transition from a high-authority control mode, such as that used for stationkeeping maneuvers, with its relatively high angular rates, to a low-authority control mode, which relies on a momentum-bias attitude control system using for example a momentum wheel. The transition from the station-keeping mode to on-orbit mode must be performed carefully to avoid saturation and possibly loss of attitude control in the low-authority-on-orbit control mode, since the high angular authority on-orbit rates prior to transition can cause large nutation.
The desired control accuracy of a three-axis stabilized spacecraft is 0.01 degrees to 0.05 degrees, exclusive of pointing error due to misalignments and structural or thermal distortion. However, the typical uncorrected attitude error following a stationkeeping maneuver is a nutation circle with a radius on the order of twenty-five times greater, i.e., 0.25 degrees to 1.25 degrees. To achieve a stable on-orbit mode, nutation amplitude must be sharply reduced, and preferably reduced to zero, before control is passed to the momentum-wheel-based on-orbit controller.
2. Description of the Prior Art
Most nutation correction systems are open-loop control systems based on calculation, rather than measurement, of critical parameters. Respecting nutation control systems, the following patents were uncovered in a search of the public records of the U.S. Patent and Trademark Office.
U.S. Pat. No. 3,866,025 (Cavanaugh) discloses a spacecraft attitude control system for producing thruster firings which align the total angular momentum vector with the desired orbit normal vector while simultaneously adjusting orbit. The invention employs a commonly-available sensor and thruster system. However, it seeks to perform an orbit adjustment maneuver in a manner that is intended to minimize build-up of nutation during the maneuver, rather than provide a mechanism for correction following the maneuver. The control system computes angular rates rather than directly sensing the rates. In addition, the patent is silent about the specific transition attitude control techniques of the present invention.
Other patents uncovered which contain additional information on the general topics of nutation attenuation, correction in spacecraft systems and the like are as follows:
______________________________________ U.S. Pat. No. Inventor ______________________________________ 3,643,897 Johnson, Jr. 3,937,423 Johansen 3,944,172 Becker 3,984,071 Fleming 3,997,137 Phillips 4,023,752 Pistiner et al. 4,174,819 Bruederle et al. 4,370,716 Amieux 4,386,750 Hoffman 4,521,855 Lehner et al. ______________________________________
The Johnson, Jr. patent pertains to a spin-stabilized spacecraft, rather than a three-axis stabilized spacecraft as described herein. It attempts to keep the nutation angle bounded, but it fails to suggest how to reduce nutation to zero.
The Johansen patent describes an attitude control system for a three-axis stabilized spacecraft. It lacks any mechanism for sensing yaw angle, yaw angular rate, and roll angular rate.
The Becker patent concerns spacecraft without internal momentum wheel control and therefore is not affected by nutation.
The Fleming patent concerns the damping of a nutation circle by means of two thruster firings which are based on sensing the spacecraft roll angle only. There are no means for sensing roll, yaw, and yaw angular rates. As a consequence there can be no assurance of stability.
The Phillips patent, the Pistener patent and the Amieux patent relate to spin-stabilized spacecraft, not three-axis stabilized spaceccraft and are not relevant to the invention.
The Bruederle et al. patent pertains to attitude control of a three-axis stabilized spacecraft, but does not address the issues of nutation control and transition from the thruster control mode to the wheel control mode. There are no sensors for yaw angle or yaw angular rates.